1. Field of the Invention
The present invention relates to an image forming apparatus, a method of controlling a fixing device, and a device and method for detecting abnormality of the fixing device.
2. Description of the Related Art
As an image forming apparatus, such as a copying machine and a printer, there recently appeared on the market a color image forming apparatus capable of forming a color image on a sheet. A known fixing device for use in the color image forming apparatus includes a fixing roller having an elastic layer formed on a surface thereof. The fixing roller is heated until the temperature of the surface thereof reaches a suitable temperature (fixable temperature) for fixing a toner image on a transfer material. However, the fixing roller has a large heat capacity, and hence it takes a longer time period (warm-up time) to heat the fixing roller until the surface of the fixing roller reaches the fixable temperature.
On the other hand, as a fixing device which makes it possible to reduce warm-up time, there has been known one using a fixing film. The configuration of the fixing device using the fixing film will be described with reference to FIG. 14. FIG. 14 is a longitudinal cross-sectional view of essential parts of the conventional fixing device using the fixing film.
As shown in FIG. 14, the fixing device is comprised of a fixing film 911 in the form of a hollow cylinder and a pressure roller 914. The fixing film 911 is supported on a supporting member 915. A heater 912 is disposed within the hollow of the fixing film 911 and is fixedly attached to the supporting member 915. A temperature sensor 913 is mounted on the heater 912, for detecting the temperature of the heater 912.
The pressure roller 914 is urged by a predetermined pressing force against the heater 912 such that the fixing film 911 is sandwiched between the pressure roller 914 and the heater 912, and a nip N is formed between the pressure roller 914 and the fixing film 911 so as to nip and convey a transfer material P carrying an unfixed toner image t formed thereon. The pressure roller 914 is driven by a drive motor (not shown) for rotation in a direction indicated by an arrow in FIG. 14, and the rotation of the pressure roller 914 causes rotation of the fixing film 911 such that the inner surface of the fixing film 911 is moved in sliding contact with the lower surface of the heater 912 while being guided by the supporting member 915.
The unfixed toner image t is heated and pressed while the transfer material P carrying the same is passing through the nip N, and is fixed on the transfer material P.
The fixing film 911 is formed of a heat-resistant resin endless film, for example, which has a thickness of approximately 500 μm, and has a surface on which is formed a release layer (e.g. a fluorocarbon resin coating layer) having a thickness of approximately 10 μm. The fixing film 911 has no elastic layer formed thereon, so as to reduce the heat capacity of the same.
The heater 912 is comprised of a ceramic substrate and a resistive heater element formed on the ceramic substrate. A temperature detected by a temperature sensor 913 mounted on the heater 912 is input to a controller (not shown). The controller performs temperature adjustment control for adjusting the temperature of the nip N to the fixable temperature, based on the temperature detected by the temperature sensor 913. More specifically, the temperature of the heater 912 (i.e. energization of the heater 912) is controlled.
In the fixing device constructed as above, since the heat capacity of the fixing film 911 is set to a very small value, it is possible to raise the temperature of the nip N to the fixable temperature in a short time after the heater 912 is energized.
However, the fixing device using the fixing film 911 having no elastic layer is not suitable for a color image forming apparatus. When the fixing film 911 having no elastic layer is used, the surface thereof cannot come into intimate contact with recessed portions formed on the transfer material due to unevenness of the surface of the transfer material, irregularities caused by the presence/absence of a toner layer, and unevenness of a toner layer. As a consequence, the amount of heat applied to the surface of the transfer material via the fixing film 911 differs between projecting portions and recessed portions on the transfer material. More specifically, since the projecting portions come into intimate contact with the fixing film 911, the amount of heat transferred to the projecting portions from the fixing film 911 is larger than that of heat transferred to the recessed portions.
A color image is formed by superimposing toner layers of a plurality of colors one upon another, and hence unevenness of the color toner layer of the color image thus formed is larger than that of the monochrome toner layer of a monochrome image. Therefore, when the fixing film has no elastic layer, gloss unevenness of the fixed image is increased, which causes degradation of image quality. Further, when an image is formed on an OHP sheet, transmissivity of light through the OHP sheet used for projection is degraded, which is likely to cause degradation of image quality.
To solve this problem, there has been proposed a fixing device using a fixing belt (fixing film) formed with an elastic layer (see Japanese Patent Laid-Open publication (Kokai) No. 11-15303).
However, a material, such as silicone rubber, used to form the elastic layer of the fixing belt, has a high thermal conductivity and is easily deformed. As a consequence, if the fixing belt is left under pressure for a long time, the elastic layer does not sometimes return to its original state from the deformed state during a warm-up time period before the start of printing, which prevents a proper amount of heat from being transferred from the fixing belt to a toner image on a sheet. This causes a change in the surface reflectivity of a fixed image, leading to degradation of image quality.
Further, in order to prolong the service life of the fixing belt, it is required to provide a unpressurizing mechanism for automatically stopping the application of pressure to the fixing belt when the image forming apparatus enters an energy-saving mode or when the power of the same is turned off. The unpressurizing mechanism separates the fixing belt from the pressure roller to thereby stop the application of pressure to the fixing belt.
However, if the heater is energized when the fixing belt is in an unpressurized state due to occurrence of some abnormality, heat is not transferred to the pressure roller, and hence the temperature of the elastic layer of the fixing belt rises sharply. As a consequence, the heat damages the elastic layer, which leads to reduction of the service life of the fixing belt. Further, the sharp rise in the temperature of the heater can cause the breakage of the heater.
To solve this problem, there has been proposed a fixing device including a power cut-off circuit configured to detect an unpressurized state of a fixing belt and forcibly cut off supply of electric power to a heater (see Japanese Patent Laid-Open publication (Kokai) No. 2005-321511).
However, in the fixing device having the power cut-off circuit, even though the fixing belt is in the unpressurized state in actuality, this state of the fixing belt is sometimes not detected due to occurrence of some abnormality. In such a case, forcible interruption of supply of electric power is not executed by the power cut-off circuit despite the unpressurized state of the fixing belt. If electric power is supplied to the heater in such a state, damage is caused to the fixing belt and the heater.